So everyone knows, if you edit your sig and do not close your marquee tag, it is very likely to corrupt your sig editor.
This means close your <marquee> tag with a </marquee>

Don't leave the </marquee> out and you should be fine.
In the case that your sig editor is somehow corrupted, please post here and a SMod or above will fix it for you.
Also note, if your sig is extremely long, you could have hit your sig's physical limit. In this case your profile will explode in a fiery ball of plasma while you watch the forums melt before your eyes as they get sucked into the singularity you created.
... Or, more likely, it will cut off your </marquee> tag.

So don't create a crazily long marquee, hehe, or it may destroy your sig editor.

If this is the case a SMod can also help you. If after adding the </marquee> tag it still doesn't appear to change, he/she will need to remove a section of the sig to make space for it. The removed section would be PMed to you.
Hope this is somewhat enlightening, lol... Sorry about the confusion.

As you all know, a somewhat common problem for members involves "broken signatures", which can warp pages for the member of the signature, and everyone viewing a page which has a post by that members. Not to mention that the sig doesn't always look all that good.

Ladies and gentlemen, I present... the Open Tags indicator.
Located conveniently under the button for inserting hyperlinks, the Open Tags indicator is a useful little thing that does just what you'd glean from the title: It tells you how many tags might be open in your post/signature/PM/etc.
One of the most common problems with "broken signatures" is an open tag. Just one tag that is failed to be closed, especially when it comes to more complicated things such as marquee, can have disastrous results, as stated earlier. Though it can be somewhat... erratic at times, it's a nifty little feature, and I suggest that everyone take advantage of it.
So remember kids, when your sig is scrolling across the screen like a caffinated gazelle, what do you do?
Yes, that's right - Check the Open Tags indicator!

"Knowledge is power."
- William Alexander, Earl of Stirling